1. Field of the Invention
This invention relates generally to electrostatic transducers and in particular to an electrostatic transducer which has improved electrical and acoustical properties.
2. Description of the Prior Art
Electrostatic transducers according to the prior art require a driving power source and a high voltage DC source for applying a DC bias. These devices utilize a high voltage DC power source which is complicated in construction. For example, generally a thin high polymer film is used as the vibrating plate; and in order to obtain electrical conductivity on the vibrating plate, a metal film, as for example of aluminum, gold, titanium, or other suitable metal, is formed on the high polymer film by vacuum evaporation, or alternatively, a surface active agent is coated on to the high polymer film. This makes the vibrating plate very expensive and also adds weight to it so that it becomes very heavy. The high voltage DC source is usually obtained by rectifying commercially available AC power. An oscillator driven by a battery can be used to obtain a high voltage DC output; or alternatively, a vocal signal can be rectified to obtain a DC signal. However, high voltage DC sources require a number of circuit elements and require a separate voltage source thus resulting in safety, maintenance, cost and other problems.
In order to avoid these defects at the present time, transducers utilizing electrets as the diaphragm are widely used. The electret diaphragm requires no DC voltage source circuit; and thus, the entire circuit can be simplified. A conductive layer is formed on one surface of the electret (high polymer) film and a capacitor is formed by the conductive layer and a back electrode between which the electret film is clamped and the diaphragm or electret film becomes heavy due to the conductive layer formed thereon. This results that the electro-acoustic high frequency response does not have high fidelity.
Recently, a uni-electret diaphragm formed by charging a high polymer film with a monocharge has been proposed. This type of diaphragm is very light because it requires no conductive layer; and thus, the electro-acoustic conversion can be performed with high fidelity over a wide frequency range from low to high frequencies with such device. However, since the uni-electret diaphragm carries a monocharge, it is strongly affected by external charges and its stored charge will also be rapidly reduced as compared with a diaphragm having a conductive layer. Thus, transducers of the prior art utilizing uni-electret diaphragms have very low electric-acoustic conversion efficiency after being used for long periods of time.